Numerous reconstituted tobacco compositions and processes for their manufacture are known, in which tobacco particles are formed into a coherent integral structure such as a rod or sheet which is thereafter used as binder or wrapper in cigars or as filler in cigarettes or cigars. The reconstituted structures desirably also exhibit strength and selective surface properties for aesthetics and handling, as well as required flexural properties for processing through tobacco machinery, rendering formulation a critical aspect of manufacturing operations.
Conventional methods for the preparation of tobacco sheet from comminuted tobacco employ a relatively low viscosity low consistency aqueous slurry of tobacco and an adhesive which is cast on a supporting surface and dried. Conventionally, these slurries remain castable only up to about 9-11% solids. Such methods are naturally energy intensive with regard to the necessity of removing relatively large quantities of water. The low-solids slurrying technique has been deemed necessary, however, because of the difficulty in wetting and uniformly dispersing tobacco at high solids, and the unavailability of readily dispersible agents effective as adhesives and handleable at high solids levels.
Improvements have been made in these arts to provide high solids processable slurries, as described in copending and commonly assigned U.S. Pat. No. 4,144,894 to Schmidt, et al and incorporated herein by reference; and improved mixing techniques at high solids levels, as described in copending and commonly assigned U.S. application Ser. No. 001,249 of Schmidt filed Jan. 5, 1979 also incorporated herein by reference.
Thus, high solids slurries may now be employed to reduce the energy considerations in such processes, but further improvements have been sought. One objective in the preparation of any reconstituted tobacco sheet is adequate strength i.e., tensile and tear properties should be sufficient to prevent cracking, crumbling, tearing or stretching in processing and handling.
Tobacco sheet of enhanced tensile strength has been reported in U.S. Pat. Nos. 2,897,103; 3,097,653; or 3,115,882 to be obtainable by careful control over tobacco particle size in dry or wet grinding. As tobacco constitutes at least 75 percent by weight of the sheet, it is more desirable and less energy intensive to control tensile strength by other means.
Certain tensile and tear properties are afforded by ensuring adequate cohesiveness and flexibility in the sheet, as by the selection of appropriate adhesive agents. Generally, however, adhesives alone have not been able to supply the full measure of strength, tear resistance and resistance to disintegration under the range of tobacco processing conditions to which the reconstituted tobacco sheets are subjected. For example, in cigarette sheet applications, fiberless formulations generally have impaired shreddability resulting in more breakup during shredding and shorter shreds. This, in turn, adversely affects the filling power of these shreds (i.e., the firmness contributed to cigarettes by a unit weight of these shreds).
Accordingly, refined softwood cellulosic fiber has been employed to reinforce the adhesive system in the reconstituted tobacco sheet, thus increasing the tensile strength, flexural strength and resistance to disintegration. Softwood cellulosic pulps in the unrefined or lightly refined condition have cellulosic fibers which are relatively long and free to interengage and entangle, causing agglomerations which result in slits and other difficulties during the process of casting thin films from reconstituted tobacco slurries. In addition, long fibers tend to orient in the machine direction during casting, providing a large difference in strength characteristics in the longitudinal and transverse directions (i.e., a large orientation factor), which is undesirable in some applications. The further refining of softwood cellulosic pulp can reduce the fiber length to a point where it does not interfere with the casting of thin films. However, the mechanical work input during the refining operation fibrillates the cellulosic fibers into a branching network of smaller and smaller fibrils, which results in an interlocking network in the final tobacco sheet. This network of fibers and fibrils is largely responsible for improved physical properties in the reconstituted tobacco sheets, but an undesirable consequence of the refining operation on softwood pulp is the increase in viscosity of the fibrous mass as the pulp becomes more fibrillated and hydrated. When such pulps are added to reconstituted tobacco slurries they result in substantial increases in viscosity. This, in turn, necessitates slurry preparation at lower solids so that the mass is still formable into thin films, resulting in increased drying costs to remove the extra water added. In addition to the increased viscosity and higher drying costs associated with refined softwood pulps, capital costs in the plant are increased due to the requirements for a paper refining system, and labor and utility costs are increased for the pulp refining operation and the energy costs associated therewith. U.S. Pat. Nos. 3,125,098 and 3,464,422 describe the preparation and use of very highly refined pulps in tobacco sheet manufacture to enhance tensile strength and to reduce orientation factors associated with more coarsely refined pulp. However, although the products obtained are superior, the high degree of refining produces pulps with even higher viscosities which must be processed at even lower solids (i.e., with increased drying costs). The longer refining cycle for such pulps increases the energy input required for refining and its labor content.
Accordingly, it is an object to provide reinforcement to reconstituted tobacco sheets.
It is also an object to employ a fibrous reinforcing agent which is compatible with the casting system in use.
In addition, it is an object to provide such fibrous reinforcement without substantially increasing the viscosity of the tobacco slurry.
Further, it is an object to provide fibrous reinforcement which is compatible with high solids castable tobacco compositions.
It is another object to prepare tobacco sheet products with adequate physical properties at minimum capital and operating expense.
These and other objects are achieved in the practice of the present invention as set forth in the following description.